In view of the acknowledged toxicity of benzene and olefins, unsaturated compounds, the general tendency is to reduce the content of these constituents in gasoline.
Benzene has carcinogenic properties, and it is therefore necessary to restrict to a maximum any possible pollution of the ambient air, in particular by excluding it in practice from automotive fuel. In the United States, reformulated fuels must contain no more than 1% benzene; in Europe, even though the requirements are not yet as strict, recommendations are gradually veering towards this value.
It has been acknowledged that olefins are among the most reactive hydrocarbons in the cycle of photochemical reactions with nitrogen oxides occurring in the atmosphere and resulting in ozone formation. An increase in the concentration of ozone in the air can be the cause of respiratory problems. It is therefore desirable to reduce the content of olefins in gasolines, and, more particularly, the content of lightest olefins which are most likely to become volatile when fuel is being processed.
The benzene content of a gasoline is very largely dependent on that of the reformate component of that gasoline. The reformate results from a naphtha catalytic treatment, the aim of which is to produce aromatic hydrocarbons comprising mainly from 6 to 9 carbon atoms in their molecule and whereof the very high index number imparts antiknock properties to the gasoline. As a result of the toxicity mentioned hereinabove, maximum reduction of the benzene content in the reformate is necessary. Several methods can be envisaged.
A first method consists in limiting the content of benzene precursors, such as cyclohexane and methylcyclopentane in the naphtha constituting the charge to a catalytic reforming unit. This solution is effective in permitting a substantial reduction of the benzene content in the effluent of the reforming unit but is not enough by itself when it is a question of reducing the content to as little as 1%. A second method consists in eliminating, by distillation, a light fraction from the reformate containing benzene. This solution results in a loss in the order of between 15 and 20% of the hydrocarbons which would be otherwise valorisable in gasolines. A third method consists in extracting the benzene present from the effluent of the reforming unit. Several known techniques are applicable in theory: solvent, extractive distillation, adsorption. None of these techniques is used on an industrial scale because none of them permits economical selective extraction of the benzene. A fourth method consists in the chemical conversion of the benzene into a constituent free from legal restrictions. Alkylation using ethylene converts the benzene mainly into ethylbenzene. However, this operation is tedious because of the intervention of secondary reactions which require separation operations which are costly in terms of energy.
The benzene in a reformate can also be hydrogenated into cyclohexane. Since selective hydrogenation of the benzene is impossible in a mixture of hydrocarbons which also contains toluene and xylenes, it is therefore necessary to first of all divide up that mixture in order to isolate a cut which contains only benzene and which can thus undergo hydrogenation. A process has also been described wherein the hydrogenation catalyst of the benzene is included in the stripping zone of the distillation column which separates the benzene from the other aromatics (Benzene Reduction--Kerry Rock and Gary Gildert CDTECH--1994 Conference on Clean Air Act Implementation and Reformulated Gasoline--October 1994.), which permits savings in respect of apparatus.
The hydrogenation of the benzene in a reformate results in a loss in the octane number. This loss in the octane number can be compensated for by adding compounds with a high octane number, e.g. ethers such as MTBE or ETBE, or branched paraffinic hydrocarbons. These branched paraffinic hydrocarbons can be generated by the reformate itself, by isomerisation of the linear paraffins. However, it is known that isomerisation catalysts of straight paraffins into branched paraffins are not inactive with respect to hydrocarbons of other chemical families. Of those which distill with benzene as a result of the azeotropic phenomenon, cyclohexane is converted partly into methylcyclopentane, for example. This reaction of naphthenic products competes on the catalyst with the isomerisation reaction of the paraffins and thus decreases its progress. On the other hand, isoparaffins with 7 carbon atoms per molecule undergo cracking which results firstly in gradual coking of the isomerisation catalyst and therefore in reduced activity and secondly in a reduction of the yield of the desired product, that is to say of the light reformate for inclusion in the gasoline.